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Rooftop packaged HVAC units (RTUs) serve over 40% of commercial building space with relatively reliable air conditioner and furnace operation; however, improper settings and failed controls–especially economizers–negatively impact indoor air quality and energy use. A field survey of over 300 packaged units found 91% with at least one problem and 64% with two or more problems. To date, utility HVAC programs, codes, and green standards have focused on higher efficiency units or unit tune-ups to the exclusion of ventilation and control upgrade opportunities. Hourly simulation over a range of U.S. climates shows that a comprehensive package of RTU control retrofits produced HVAC savings between 30% and 48%–ten times the savings from incrementally higher efficiency unit replacement. The proposed RTU control retrofits make economizers more reliable, improve ventilation control, and reduce energy use. The retrofits include stand-alone direct digital controls, demand controlled ventilation with integrated fan cycling or fan speed control, premium economizer control, and ventilation lockout during warm up. Field testing showed that for RTUs, a single return air carbon dioxide sensor was as effective as multiple-room sensors in allowing demand controlled ventilation to improve indoor air quality. Field testing of four RTUs with variable speed fans and solid-state DCV controllers was extended to include six additional units that are using DDC controls with fan cycling rather than fan speed reduction. The fan cycling control algorithms are demonstrated to comply with ASHRAE Standard 62.1.

Citation: ASHRAE Conference Papers, Las Vegas, NV